THE end-all FIX for the Heathkit AA-100 integrated amp.

Well after pm'ing the master on this model I found the thread I knew existed and here it is!

I have been know to dabble around on Heathkit gear from time to time. It might have been Gordon who I once read referred to them as "Griefkits". Yep they can be.

Anyhow I recently came across a mint, yes mint Heathkit AA-50. You folks may recall earlier this year I picked up a pair of Heathkit EA-2s they were never finished by the original builder. One did not work and the other was still in parts. Seem Uncle Joe, or who ever, made a critical mistake on the first one, never got it to work, and therefore just gave up.

This AA-50 was in same situation. Several critical steps were not done when whoever build this thing many moons ago tried his/her hand at kit building. I actually found the main grounds to the second filtering cap just laying underneath one of the resistors attached to the cap. Never pulled thought the ground lug and solder, just laying they for 50 plus years waiting for someone to find them.

Anyhow. Now that I found this thread I will get going on the mods and get this thing up to full potential. You can see in the provided pictures just how clean this unit is.

I have already replaced the filter caps, installed new Russian caps for the output tubes, and will get on with the rest of restoration. Restoration may not be the correct word in this case. This fricker is 50+ years new now.

There was one broken part. One of the lunar lander style feet was broken. I have plenty of spares.

Inspired by this thread (thanks Gordon), I've just got a nice 'for parts' AA-50. It has been well used and comes complete with a dead 7591 and 7199, missing fuse holder and power switch, but still looks reasonable cosmetically. The power switch may have been removed because it needs a step down transformer here, 240v to 117V. Lots of checks before I even think of powering it up.

I intend following the rebuild steps here, so no need for a separate thread, I think. I'll post some pics when I get it home.

A couple of fuzzy phone pics of my AA-50. Not as clean as Dan055's, but not bad. After replacing the fuse, bias diode and missing tubes, I couldn't resist seeing if it was basically working. It is, but distorted and blew the fuse after a minute.

I recapped the power amp tonight and replaced the four 470K grid resistors with 220K ones as Gordon advises. It sounded MUCH better, but blew the replacement fuse quite quickly. I suspect the first 60/20 can cap, and will replace it next. The first 60/25 cap measures 105/90 on my capacitance meter. After replacing it (any maybe the other can) I'll do some proper voltage measurements.

I've got the other caps lined up for the preamp board and look forward to getting to know this amp.

Just a quick update: both can caps had to be replaced (as the AA-50 is older than the AA-100) as well as the selenium bias rectifier (and cap). All the caps on the preamp board too. Thanks again for advice on restoring these unregarded amps. Of course, if anyone were to produce replacement PCBs, that would be great too!

Sorry, it happened again! I posted two questions about the purpose of the 6.8k and 2.2k resistors and the effect of the new bias arrangement on B+, but both are already answered in posts #40 and 44 of this excellent thread.

Later: After getting the bias sorted out, I was still puzzled by distortion in my amp. After isolating it to the preamp, and comparing all the voltages with those on the schematic, I realised that MANY of the resistors must be way out of spec--and they were. After replacing all of these (about two thirds of the board), and dropping the B+ via a 120 ohm 20W resistor, it's sounding good. I thought it is worth adding this point, as this AA-50 has needed to have many more resistors replaced than any other vintage amp I've worked on.

New member to the group and just finished my Heathkit AA-100. Got it in terrible condition, cherry tubes and all. Found broken traces, etc. Now working well. Will replace AX7's with AT7's soon. It's coming along great.

New member to the group and just finished my Heathkit AA-100. ... How warm is the power transformer supposed to be to the touch?

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Welcome and well done! This is an incredibly helpful thread for saving Heathkit AA100s and AA50s.

Care to post a picture of your work above and below? Sorry I can't answer the question about your PT as mine's in storage and works on UK mains via a step down. I seem to recall it got quite hot, a bit like a Dynaco ST70.

PT heat is always a concern to me. After some time with the AA-100, I guess it feels normal. Yes, they seem to run a bit hot. My current dilemma is with replacing the AX7 tubes with AT7. Not sure what to expect; a noticeable difference?

12AT7s subjectively improve the dynamics, and seem also to make the sound "smoother" at the same time.

I think it's basically that under the load in that preamp circuit (with 100K plate resistors), the 12AT7 just is in a more linear part of its operating characteristics (especially at high output levels), than the 12AX7 is, in this case.

Ok, so the 12AT7's were installed in the AA-100 and it does seem to have a richer sound. My imagination? Still have the power supply caps to replace. All in all, this project is turning out great. Been feeding it with a Sparta broadcast turntable. Real vinyl!

12AT7s subjectively improve the dynamics, and seem also to make the sound "smoother" at the same time.

I think it's basically that under the load in that preamp circuit (with 100K plate resistors), the 12AT7 just is in a more linear part of its operating characteristics (especially at high output levels), than the 12AX7 is, in this case.

Regards,
Gordon.

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It appears the 7591's I bought are a bit too tall for the top cover to clear. May run it without a cover at the cost of tubes.

Ideally print out a couple of copies on large A3 sheets, and then go through each channel, checking the voltages against what the schematic suggests. I find it's easier to write down the measurements first, before trying to figure out where a problem might be.

Once you are measuring DC, clip the negative (black) probe to the chassis and take measurements with the red (pos) probe. Only use one hand and keep the other in your pocket for safety.